Many processes have been used to liquefy the constituents of air.
U.S. Pat. No. 3,605,422 discloses a process for liquefying nitrogen in which nitrogen from a medium pressure column is compressed by two compressors in series and then divided in three portions. Two of the portions are respectively expanded to the same pressure in a turbine, one portion being expanded in a cold turbine and the other portion being expanded in a warm turbine. The remaining portion is liquefied and sent back to the column.
U.S. Pat. No. 4,778,497 relates to a process in which the nitrogen gas to be liquefied is compressed in two boosters in parallel to the same pressure and divided into three portions, two of which are expanded at different temperatures to the same pressure and one of which is liquefied and expanded in a turbine.
U.S. Pat. No. 4,883,518 discloses a process in which medium pressure nitrogen is compressed by two boosters in series and then divided in two, one part being liquefied and sent back to the column and the other being expanded in a cold turbine and recycled to the booster. A portion of nitrogen not compressed by the boosters is expanded in a warm turbine to the same pressure as that of the outlet of the cold turbine.
U.S. Pat. No. 4,894,076 concerns a more complex process using at least four turbines.
In U.S. Pat. No. 5,231,835, nitrogen is compressed by two boosters in series and sent to a warm turbine. The cold turbine is fed with nitrogen which does not pass to the boosters and produces an expanded nitrogen stream at a lower pressure than that produced by the warm turbine.
This arrangement has the advantage of reducing the pressure ratio across the cold expander in order to keep a good efficiency on this turbine. Nevertheless, this is not the best arrangement in terms of efficiency of the liquefying process.
It is well known in the art that it is always better to have the higher pressure at the inlet of the cold turbine in order to keep a low pressure ratio, which means increasing the outlet pressure of the cold turbine. The problem, presented in U.S. Pat. No. 5,231,835, in which the cold turbine has a high outlet temperature (due to the higher pressure of the cold turbine, which is due to the higher pressure resulting in a higher temperature of the cooled supercritical fluid after heat exchange with the cold turbine outlet) can be solved by incorporating the present invention which relates to the subcooler design.